Linearization of load cells



LINEAR-NATION 0F LOAD CELLS Filed, Dec. 24, 2 Sheets-Sheet 1 ov OUTPUT MILLIVOLTTS/ VQLT F IG. 2.

FIG-4.

ALFRED N. ORMOND A T7'0RNEYS Jan. A N. ORMOND LINEARIZA'II'ON OF LOAD CELLS Fil'd Dec. 24, 1963 v I I z Sheets-Sheet 2 INVENTOR.

ALFRED N. ORMOND BY WA fiaazmqa Y Arrazlvixs United States Patent 0 Filed Dec. 24, 1963, S81. No. 334,082

, 11 Claims. oms- 141 This invention relates generally to load cells and more output signal from such load cells to the end that greater accuracy is provided.

This application is a continuationdn-part of my copending application: Serial No. 112,341, filed ,May 24,

1951, and entitled System for Linearization of Load Cells. now abandoned and of my co-pend'ing application Serial No. 208,869, filed July 10, 1962, and entitled Means for 1 Linearization of Load Cells, now abandoned.

Conventional load cells usually comprise a deformable member or spring element incorporating strain gauges 'for providing an electrical output signal. This signal is a function'of the deformation of said member as a consequence of a load. applied to the member. The value of the output signal may be calibrated forvarious known loads applied to the cell toprovide a proper scale. The load v cell may then be employed in other environments to measure specific loads.

In present day load cells, however, theoutput signal is not linear with a change in load. This is a consequence of extraneous signal.. 7

Such extraneous strains result from physical changes in strains affecting the linearity of the output the loaded] itselfunder deformation, inability to provide a constant current in the electrical bridge portion of the ercuit constituting part of the signal transducer means, temperature variations, and non-linearitiesin the particular strain gauges employed. if these extraneous strains could be compensated for, a more accurate output signal could beprovided.

In the first mentioned of my foregoing patent applica tions, compensation was achieved by providing additional strain gauges of the conventional wire or foil types connected to the cell to provide further signals used to compensate the extraneous signals from the principal strain gauges. However, I have now found that inclusion of these types of strain gauges may introduce a large resistance in the input circuit thereby decreasing the etfective voltage across thebridge. The output reading as expressed in millivolts per volt input is therefore appreciably less for maximum or 100 percent loading, thereby contributing to the non-linearity of the signal to be linearized.

' With the foregoing in mind, it is a primary object of this inventionto provide a load cell system in which a linear output signal is realized over substantially the entire range of loading for which the cell is designed. More particularly, an object is to provide an improved load cell system' incorporating improved compensating means for linearizing the output signal by compensating for extraneous strains in the system without introducing large resistances in the input circuit.

Briefly, these and other objects of this invention are attained by employing auxiliary compensating strain gauge means or transducers to generate a compensating signal in response to loading of the cell by the principal load. Thes'egenerated signals, in turn, are electrically combined with the conventional output signals from the measuring particularly to an improved means for linearization of the E when plotted against the force F will result Patented Jan. 11,1966

strain gauges in such a manner as to cancel the undesirable extraneous efi'ects produced in the principal signals so that the resulting net output signal is substantially linear.

The use of semi-conductor type strain gauges is preferable in that the desired linearization can be achieved without introducing large resistance in the input circuit.

A better understanding of the invention will be had by referring to an exemplary embodiment thereof as shown in the accompanying drawings, in which:

FIGURE 1 is a schematic diagram showing a. simple type of load cell incorporating transducer means in accordance with the present invention; I

FIGURE 6 shows a fourth bridge circuit arrangement.

Referring first to FIGURE 1, there is shown a load cell in the form of a deformable cylindrical member 10. As shown, there are provded first transducer means including a first pair of strain gauges T and T connected on diametrically opposite portions of the member 10. These gauges change their resistances in response to tension forces developed as a consequence of an applied load in-' dicated by the arrow F in FIGURE 1. The first transducing means also includes a pair of strain gauges C and C oriented to change their resistancesin response to compression forces resulting from the applied load force F.

In the absence of further transducer means, the output signal from the transducers will be non-linear as indicated by the curve 11 of FIGURE 2. One reason for this nonlinearity, for example, is the fact that the applied force results in an actual physical change in the cross-sectional area of the load cell. For example, compression of the cylindrical member 10 of FIGURE 1 will tend to circumferentially expand the member as a consequence of physical displacement of the matter in the cylinder. Such action will increase the cross-sectional area indicated by A in dotted lines. Since thestress developed is a function of the force divided by this area, the stress then becomes a function of the force divided by the area plus a small incremental change in the area. This relationship is indicated by the formula in FIGURE 2 showing that F itself will not be linear as a consequence of the physical change in the area of the member 10. Thus, the strain e which is equal to the stress divided by Young"s modulus of elasticity in a nonlinear curve 11.

Other factors aside from the small incremental change in the cross-sectional area of the load cell 10 will result in non-linearity of the output signal. For example, the electrical characteristics of a strain gauge bridge into which the strain gauge members are connected are nonlinear because it is not possible to realize a perfectly constant current in the arms of the bridge. Also, the physical properties of the strain gauges themselves are non-linear and temperature sensitive. mation will not always result in a consistent change in the resistance over the range of the gauge.

In other words, a unit defor-' The non-linearity. caused by the various extraneous To overcome or compensate for these'extrancous strains in accordance wtih the present invention, there are provided novel additional transducer means which take the nected to beresponsive to the-loading P so thattheir resistance values willalso change with changes in the load- ".form of additional strain gauges indicated at R and R ing-F. These additional strain gauges are then connected into the bridge circuitlor the principal strain gauges. as

, illustrated in FIGURES.

:With particular reference toFIGURE 3, there isshown a strain gauge-bridge 13 wherein the tension strain gauges T and T are connected in opposite arms and the compression strainganges C and C are connected in the remaining opposite arms. These arms are connected together to define vertices 14, 15,16 and .17.- Op- 7 p'osite' vertices 14 and 16 areconnected Qthrough leads 18 and '19 to a source of voltage V to define an input diag-' onal of the bridge. Opposite vertices T and 17, on the.

other hand, connect-through output leads andllto a suitable indicator or recorder V to define an output diagonal of the bridge. Anexample of an indicator would be a'simple vacuum tube voltmeter to provide a reading.

of the V as indicated in FIGURE .3

Whenthe voltage source is of low impedance, the sec- 0nd transducer means-providing the desired compensation in the form .of the additional strain gauges R and R are connectedinseries in the input diagonal between the bridge circuit 13 andthe'volta'gesource"V Small trim- In'ing resistancessuch as indicated at r may :be shunted across vR and R to enable adjustmentof the compensating gauges."

If. a high impedance source is employed rather than a 'low impedance source, the compensatingtransducer means in the form of a strain gauge R and series trimming resistance r would. be connected across the source input leads '18. and '19 as shown in FIGURE 14. to provide a parallel path in the input diagonal for current supplied to the bridge.

Similarly, ifa low impedance indicator or recorder is used at the output, seriesgcompensation .can be provided in the output diagonalleads 20 -and' 21 ratherthan in the input-diagonal leads 1'8 and Y19 as s'hownat'Riand r" and R and r" in FIGURES. v

If ahigh impedance indicator or recorder .is used, the compensating means is connected in parallel with the output diagonal leads 20 and Has shownat R andr' inFI G'URE 6.

Moreover, the series type compensation could be ef-' feeted by incorporating a series connected compensating gauge in only one of the. leads-J18'aud .19, or 20 and .21 rather than .in each of the input or output leads.

R the series resistance introduced need onlybe 20 ohms,

for example, with the provision of the same magnitude compensating signals. As a result, the voltage across the bridge is notmaterially different from the input voltage so that a far more accurate .millivolt per volt'output signal is provided at theoutput. The curve 11 inJFIG- URE'Z 'is thus brought into coincidence with the desired linear curve 12.

-An'example ofonetype of semiconductor strain gauge which could be used for the correcting resistances would be silicon doped with P-type .or 'N-type impurities.

While the particular embodiment of the invention has taken the form of a cylindrical load cell as illustrated in FIGURE 1, it should be understood :that the principles are applicable to other shaped load .cells ,for measuring provide a first output signal having a nonelinear characteristic, said first transducer means comprising strain gauges arranged in an electrical v,bridge network having diagonals defining input and output connections for said bridge; a second transducer means connectedto saidimembet and responsive to deformation of said member as a consequence of said load to provide a second output'signal having a value that is a function of said lead; and means for connecting said-second transducer means to at least one of said diagonals of said bridge to provide a net output signal constituting acombination of said first and second'output signals which is linear.

2. Accll, according to claim 1, in which said'first transducer means comprises a'first pair of strain gauges changing their resistances in response to tension forces genended in said member by said load; and a second pair of strain gauges changing their resistance in response to comprcssion'forces generated in said member by said load; said electrical bridge including said first pair of strain gauges connected in two opposite arms of said bridge and said second pair of straingauges connected in the remaining opposite arms of said bridge; a source of voltage'conneeted across opposite vertices of said I bridge to define said input diagonal; anindicatingmeans In each arrangement,-.changes in the compensating re- I sistanceswill .occur and thus control the signal 'to the bridge circuit'13 or the signal at the output of the bridge. The degree of control, as mentioned, can be adjusted by the trimming resistances such as to insure'that the final outputvo'ltage'V will be absolutely linear with changes inthe' loading'F.

Sincesemi-conductorsexhibit up to fifty times the sensitivity of normal strain gauge material made of metallic conductors, it is: preferable to employ semi-conductortype straingauges in the circuitsof FIGURES 3, 4,5 and 6.

As an example ofthe-improvement afiforded by the use of semi-conductor strain gauges, consider the'circuit of'FIGUREB. If R, and R constituted the conventiona1 type of strain gauges, there would be introduced a series resistance of perhaps 1,000. ohms to provide necessary compensating signals. As a result, the actual voltage across the bridge wouldbeiconsiderably less so that the millivoltper voltoutput reading would be lcss'than connected across the remaining opposite vertices of said bridge to define said output diagonal, said second transducer means comprising correcting strain gauge means having a resistance which varies as a function of said loadingsecure'd .to said member.

3. A cell, according .toclaim 2,.including variable trimming-resistance means. connected to said correcting strain gauge means for adjusting theoverall resistance 'value of said correcting strain gauge means under any given load.

4. Acell according to claim .2, in which said correcting strain gauge means includes a semi-conductorstrain gauge.

5. A cell, according to claim 4, in which said semiconductor strain gauge comprises silicon doped with N- type impurities.

.6. A cell, according to claim 4, .in which said semiconductor strain gauge comprises silicon doped with P- type impurities.

'7. A cell, according to claim 2, in which saidcorrecting strain gauge meansis connected in series with said input diagonal.

8. A cell, according to claim 2,.in which said correcting strain gauge means is connected in parallel withvsaidinput diagonal.

9. Acell, according to claim 2, in which said correcting strain gauge means is connected in series with said output diagonal.

10. A cell, according to claim 2, in which said correcting strain gauge means is connected in parallel with said output diagonal.

11. A load cell spring element strain gauge bridge circuit for generating an output substantially linearly related to loads applied to the spring element, said circuit comprising four similar strain gauging means operatively atv tached to a surface of said spring element and connected to define a four-arm bridge network having input and outvariable series impedance in accordance with loads ap- 5 plied'to said spring element.

No references cited.

RICHARD c. QUEISSER, Primary Examiner. I 

11. A LOAD CELL SPRING ELEMENT STRAIN GAUGE BRIDGE CIRCUIT FOR GENERATING AN OUTPUT SUBSTANTIALLY LINEARLY RELATED TO LOADS APPLIED TO THE SPRING ELEMENT, SAID CIRCUIT COMPRISING FOUR SIMILAR STRAIN GAUGING MEANS OPERATIVELY ATTACHED TO A SURFACE OF SAID SPRING ELEMENT AND CONNECTED TO DEFINE A FOUR-ARM BRIDGE NETWORK HAVING INPUT AND OUTPUT DIAGONALS, AND A CORRECTING STRAIN GAUGING MEANS CONNECTED SERIALLY WITH ONE OF SAID DIAGONALS AND OPERATIVELY ATTACHED TO A SPRING ELEMENT SURFACE AND GENERATING A VARIABLE SERIES IMPEDANCE IN ACCORDANCE WITH LOADS APPLIED TO SAID SPRING ELEMENT. 